58 
MR, J. J. WATERSTON ON THE PHYSICS OF MEDIA COMPOSED OF 
densities when the air and vapour are in equal proportion at the ordinary pressure of 
the atmosphere. The vapour is that of the sulphuret of carbon employed by M. Marx* 
(‘ Scient. Mem.,’ Part 14), which has 117°'12 as its boiling temperature. With half its 
volume filled with air it assumes the ordinary pressure of the atmosphere (30 inches) 
at 80°‘55. At this point it crosses the line of the constant pressure of one atmosphere 
(see Note B) as may be seen on the accompanying chart. It may also deserve to be 
remarked that the curve crosses this line in a much more sloping direction than the 
straight line of the pure vapour, and that, consequently, according to what is stated 
in Note H, such a mixture is so much better adapted to the measurement of heights 
by the thermometer, inasmuch as 1° represents a much less difference of height in the 
atmosphere when applied to mixtures of air and a vapour, than with the same vapour 
in its pure state. We may thus, perhaps, with such mixtures, be enabled to construct 
an instrument for measuring heights by means of the thermometer which will have all 
the advantages that are anticipated in Note H from employing the pure vapour of 
a volatile liquid with a chart line of density having the smallest possible inclination 
to the axis. 
As an example of this let us take the mixture represented by the upper line of 
density, FCS. 
At the temperature 80 o, 55 and the mercury in the barometer standing at 30 inches, 
a small quantity of air saturated with sulphuret of carbon is enclosed and at the 
lower temperature, 70°, the tension of the mixture of air and vapour reduced to 
26'76 ; it is required from these data to determine the height corresponding to a 
lowering of 1 degree in that temperature which brings the tension to an equilibrium 
with the external atmosphere. 
We have first to compute G by the formula given in Note B as follows :— 
x/h 
— 1 
in which t 0 = 461 + 70, t 1 = 461 -fi 80'55, e 0 = 26‘7 6 , e x — 30‘00. 
By this we obtain G = 8 - 742. It is making use of the small arc, CS, as if it were 
a straight line. 
Vfe have next to employ the value of G in the formula of Note H, viz.:— 
3176 
y/T — ( r 
y /1 — G 
• T = h. 
In this the absolute temperature at the lower of two stations is denoted by T, and t is 
the temperature at which the tension of the enclosed air and vapour equilibrates the 
atmospheric pressure at the lower station, the same at the upper station being r. 
* [? Magnus.—R.] 
